Underground storage tanks of the above type are commonly employed for the storage of highly vaporous liquids, such as gasoline at gasoline service stations. The tanks are typically buried in the earth below the frost line. Such tanks typically have access openings through their top walls to which a riser tube is coupled and extends upwardly from the buried tank into a manhole to permit filling of the tank. A similar access opening and riser tube is provided for admitting a submerged pump device extending vertically from the riser tube to the lower portion of the tank for pumping liquid from the tank to an above ground elevation. It is also common to provide the tank with additional access openings for a vent tube and a vapor recovery line. Each of the access openings thus has a dedicated singular usage necessitating that the opening remain unencumbered with devices such as a liquid level monitoring system.
Previously, it has been known to provide such underground storage tanks with liquid level monitoring devices, however, most such known devices are mounted in one of the aforementioned access openings in such manner as to inhibit or prevent normal usage of such access opening for which it was dedicated when the level sensing system is in place. U.S. Pat. No. 4,999,900 provides a liquid level monitoring system that can be mounted in the riser pipe of the fill tube while permitting normal usage of the fill tube, but limits the type and particularly the size of the monitoring system that may be used.